In mixing calcined gypsum, particularly for use in the manufacture of wallboard, it is important that the calcined gypsum, which forms the core, be mixed so as to obtain a wallboard product in which the gypsum core is relatively lightweight. The core is comprised principally of set gypsum which has been prepared and mixed with a metered amount of water and other additives such as fibers and surfactants whereby the resultant gypsum core is of a porous or cellular structure to obtain a wallboard product which is relatively lightweight in accordance with industry practices. When the calcined gypsum slurry exits the mixer containing lumps of gypsum and the slurry is fed to a board machine for introduction between paper cover sheets, the lumps of gypsum cause the paper sheets to break which requires stoppage of the board machine to remove the broken paper sheets and/or cleanup the gypsum slurry which may spill onto the board machine through the broken sheets.
The problem of lump formation in the mixer is a long standing problem which has not been completely solved through the many years of gypsum wallboard manufacture. As disclosed in U.S. Pat. No. 2,660,416, a mixer having a self-cleaning discharge gate was developed, and in addition, scrapers were used for continuously scraping certain surfaces upon which the gypsum material would ordinarily build up. This was intended to prevent sufficient build up of material to form large lumps which if supplied to the board forming machine would cause defective boards. However, this was a continuing problem for not all of the surfaces in the mixer upon which material could build up were capable of being scraped.
U.S. Pat. No. 2,805,051 also discusses the problem of "set" plaster clinging to the walls of the mixing vessel, mixing blades and any other surfaces which it contacts, forming a hard, solid mass which clogs and otherwise obstructs the operation of the equipment.
The mixer of this invention is what is termed a "continuous mixer", i.e. one in which the ingredients are continuously fed in measured quantities and in proper proportion. The various ingredients are continuously mixed and issue continuously from the mixer as a calcined gypsum or stucco slurry for introduction between the paper cover sheets on a wallboard forming machine.
In contrast thereto, U.S. Pat. No. 4,194,925 discloses a method and apparatus for washing mixing containers with high pressure water. However, the mixing process disclosed in this patent is a batch operation wherein the washing occurs subsequent to the mixing process.
The prior art, e.g. U.S. Pat. No. 5,683,635, also teaches the use of a device in the mixer sometimes referred to as a "lump ring". It aids the mixing action in the mixer and is intended to prevent lumps of gypsum from being discharged from the mixer with the calcined gypsum slurry. The lump ring consists of two rings, one stationary and the other rotating, with a small gap (one eighth to one quarter inch) to prevent lumps from passing to the discharge gate. However, in an EHRSAM mixer, the rotating mixer teeth are external to the lump ring, and it has been discovered that lumps of gypsum form in the junction of the mixer teeth and the rotor body. These lumps are frequently dislodged into the calcined gypsum slurry causing subsequent paper cover sheet breakage.
As previously disclosed, the mixer of this invention is a continuous mixer wherein an effective proportion of water to dry calcined gypsum has been determined and a metered amount of water is fed to the mixer. In a conventional mixer, a series of low pressure water jets are used to incorporate the metered amount of water into the calcined gypsum in the mixer. In accordance with this invention, a portion of the metered water is diverted from the line leading to the low pressure water jets and is passed through a pressure washer pump or other pressurizing device to form high pressurized water. This high pressure water is fed to one or more nozzles which are directed to clean the surfaces in the mixer where material builds up to form lumps. In particular, at least one high pressure nozzle directs a spray of water at the juncture of the teeth and the rotor body. A second high pressure nozzle may also direct a spray of water against the surface of the rotating lump ring.
It is an object of this invention to provide an apparatus for mixing calcined gypsum wherein the formation of lumps of gypsum is substantially reduced by spraying high pressure water against surfaces where lumps of gypsum can form.
It is another object of this invention to provide an apparatus for mixing calcined gypsum wherein high pressure water is sprayed against the juncture of the rotor teeth and the rotor body.
It is a further object of this invention to provide an apparatus for mixing calcined gypsum having two high pressure water nozzles, with one nozzle directing a spray of water against the juncture of the rotor teeth and the rotor body and the other nozzle directing a spray of water against the rotating lump ring.
It is still another object of this invention to provide a method for operating an apparatus for continuously mixing calcined gypsum wherein a metered amount of water is incorporated into the calcined gypsum through both low pressure nozzles and at least one high pressure nozzle.
These additional objects and advantages of this invention will be readily understood from a consideration of the drawings and the following detailed description of the preferred embodiment.